I know of a guild named “The Rookery” [H-RP] The Rookery they seem like fun!
Other than that… I don’t know. BUT! I think you should not be shy hosting your own RP events and people will surely join. I think the horde is lacking a guild that accepts all races, and some people surely are hungry for RP. Keep the RP alive! Best of luck.
EDIT: Out of curiosity, I made a horde and checked the guild finder to look for RP guilds. While not strictly RP, the following guilds claim according to the description they do some RP.
1.Nazarick
2. Jadekin Tribe
3. Sylvanas Loyalists (new RP guild)
4. Theatre of Cruelty
Might be worth talking to them to see if they want to do something together.